Social norms refer to what most people do (i.e., descriptive norms) and what most people (dis)approve of doing (i.e., injunctive norms). The influence of perceived social norms and norm-based messages (i.e., messages presenting descriptive or injunctive norms) on health behaviors has long been a research focus in communication studies. However, the mechanisms that underpin social norm influence have not been fully understood. In addition, researchers have been exploring strategies to enhance the persuasiveness of norm-based messages. Based on social norm theories and the message matching theory, the dissertation focused on understanding norm conformity motivations and testing the effectiveness of norm conformity motivation appeals in changing health-related attitudes and behavioral intentions of getting a coronavirus 2019 (COVID-19) booster vaccine. By focusing on COVID-19 booster vaccine, this study aimed to extend the scope of social norm approach to crisis contexts and provide practical implications to combat the COVID-19 pandemics using norm-based message. Through a literature review, the dissertation provided a framework that synthesized norm conformity motivations identified in the literature. The framework defined five norm conformity motivations and categorized them into motivations to conform to descriptive norms (i.e., accuracy motivation, identification with admired group motivation, and relative benefit motivation) and motivations to conform to injunctive norms (i.e., social award motivation and social punishment motivation). Pilot study 1 developed and validated a 23-item instrument to measure the five motivations. Face validity, construct validity, and reliability were evaluated using Amazon Mechanical Turk (MTurk) samples. And content validity was evaluated by five expert judges. The instrument had adequate validity and reliability. Pilot study 2 designed norm-based messages with motivation appeals (i.e., linking norm (non)conformity with the benefits or costs related to norm conformity motivations). Based on the results of manipulation check, pilot study 2 determined which messages to be used in the main study. The main study compared the influence of norm-based messages and norm-based messages with motivation appeals on U.S. adults’ attitudes and intentions to get a Coronavirus disease 2019 (COVID-19) booster vaccine. The main study also examined the persuasiveness of matching norm conformity motivation appeals with individual characteristics, including norm conformity motivations, perceived uncertainty, need for closure, upward social comparison, fear of missing out, need for approval, and fear of negative evaluation. The results showed that adding norm conformity motivation appeals increased perceived message effectiveness, and in turn, perceived message effectiveness was positively associated with attitudes. However, the total effect of motivation appeals on attitudes and the mediation paths through perceived message relevance were not significant. In addition, matching motivation appeals with individual characteristics did not result in better persuasion outcomes. The study contributes to the social norm literature and health communication practice by providing a conceptual framework and an instrument of norm conformity motivations. The framework helps understand the norm conformity process. And the instrument allows future studies to empirically test the psychological mechanism of norm conformity. Health communication practitioners can use the instrument to gauge recipients’ norm conformity motivations and design tailored messages. The study also contributes to social norm theories and the message matching theory by highlighting the importance of perceived message effectiveness in norm conformity and the importance of motivation salience in message matching.